Technischer Bericht NTB 84-15
Geophysikalisches Untersuchungsprogramm Nordschweiz:Reflexionsseismik 82
As part of Nagra's regional geophysical investigations in Northern Switzerland, a first seismic reflection survey was carried out in 1981. Data on 11 lines with a total length of 182 km were recorded. The primary objective of the survey was the reconnaissance of the Mesozoic sediments of the eastern Jura and the depth of the crystalline basement surface. Of particular importance were the local structural conditions in the vicinity of the proposed Nagra drillsites. All seismic sections were processed and available for interpretation before the drilling program was started.
The information content of the data turned out to be significantly different in the two main, superimposed geological units. In the range of the Mesozoic overburden the seismic image is in general distinctive with high resolution and good continuity. Below the Mesozoic, however, the reflectivity was found to be poor and the interpretation of the data was much more difficult. Generally there are no direct reflections from the crystalline basement surface. Laminated reflections below the Mesozoic, which seemed to indicate the presence of Paleozoic sediments, occur only in certain parts of the survey area and are rarely conspicuous. It was already known on the basis of earlier geophysical data (Refraction seismic survey 81, Gravity survey 81/82) that a deep Paleozoic trough may exist just north of the Folded Jura. Locally the reflection data provided further strong and conclusive evidence (e.g. WEIACH region) but the true dimensions of what is now known as the "Permocarboniferous Trough of Northern Switzerland" were only revealed after the well RINIKEN had been drilled. Based on the drilling result and the sporadic seismic indications, it was finally possible to map the approximate shape of the trough.
The interpretation and mapping of 5 reflecting horizons led to the following conclusions: the relief of the crystalline basement surface is much more complicated than previously assumed. Its dominating feature is a 10 – 12 km wide and up to 3000 m deep through which trends in a ENE-WSW direction across the survey area. The buried northern rim of the trough approximately coincides with the outcropping Mandach overthrust; there is however evidence for dextral displacements along the WNW-ESE striking Vorwand- and Eggberg transverse faults. In the south the main trough terminates underneath the boundary between Tabular- and Folded Jura; a shallower extension possibly reaches as far south as the southernmost Jura chain. Clear vertical basement dislocations were found under all the mayor thrust faults of the eastern Tabular- and Folded Jura. They are interpreted as late Paleozoic normal faults hading towards the center of the trough. The maximum vertical displacement reaches approx. 300 m under the two main branches of the eastern Jura overthrust belt. There is sufficient evidence to assume a causal relationship between the boundaries of the Permocarboniferous trough, the basement dislocations along its southern rim and the overlaying thrust faults which initiated the formation of the Jura chains.